Materials that are used in high temperature applications, such as boilers, must have good oxidation and corrosion resistance, strength at increased temperatures and structural stability. Structural stability implies that the structure of the material during operation shall not degenerate into fragility-causing phases which lower the strength of the material. The choice of material depends on the temperature and the load, and of course, on the cost. Oxidation resistance, which is of considerable importance for the present invention, means the resistance of the material against oxidation in the environment to which it is subjected. In applications such as boilers, the environment includes the presence of high temperatures. Under oxidation conditions, i.e., in an atmosphere that contains oxidizing gasses (primarily oxygen and water vapor), an oxide layer is formed on the steel surface. When the oxide layer attains a certain thickness, oxide flakes detach from the surface. This phenomenon is called scaling. With scaling, a new metal surface is exposed, which also oxidizes. Therefore, since the steel is continuously transformed into its oxide, its load-carrying capability will gradually deteriorate.
Scaling may also result in other problems. In superheater tubes, the oxide flakes are transported away by the vapor and if accumulations of these flakes are formed, e.g., inside tube bends, the vapor flow in the tubes may be blocked and potentially cause a break-down in the boiler system because of overheating. Further, the oxide flakes may cause so called "solid particle erosion" in the turbine system. Problems caused by scaling can manifest themselves in the form of a lower boiler effectiveness, unforeseen shutdowns for repairs and high repairing costs. A reduction in scaling problems make it possible to run the boiler with a higher vapor temperature, which brings about an increased power economy.
Thus, a material with good oxidation resistance should be capable of forming an oxide that grows slowly and that has a good adhesion to the metal surface so that it will not flake off. The higher the temperature that the material is subjected to, the stronger the tendency for oxide formation. A measure of the oxidation resistance of the material is the so called scaling temperature, which is defined as the temperature at which the oxidation-related loss of material amounts to a certain value, for instance 1.5 g/m.sup.2 -h.
At increased temperature, the material is subjected to creep deformation. An austenitic basic mass, which is obtained by the addition of an austenite stabilizing substance such as nickel, improves the creep strength, as does precipitations of a minute secondary phases, such as carbides.
A conventional way to improve the oxidation resistance is to add chromium, which promotes the formation of a protective oxide layer. The alloying of chromium into steel brings about an increased tendency to separate the so called "sigma phase". This tendency may be counteracted, as indicated above, by the addition of austenite-stabilizing nickel.
Both manganese and nickel have a positive influence on the structural stability of the material. Both these elements function as austenite-stabilizing elements, i.e., they counteract the separation of fragility-causing sigma phase during operation. Manganese also improves the heat check resistance during welding, by binding sulphur. Good weldability constitutes another important property for the material.
Austenitic stainless steels of the type 18Cr-10Ni have a favorable combination of the above-mentioned properties and are therefore often used for high temperature applications. A frequently occurring alloy of this type is SS2337 (AISI Type 321), corresponding to Sandvik 8R30. The alloy has a good strength, thanks to the addition of titanium, and good corrosion resistance. Therefore, it has been used in tubes for superheaters in power plants. However, the oxidation resistance of the alloy is limited, which brings about the above-mentioned problems resulting in limitations with regard to operable life and maximum temperature of use.
Soviet inventor's certificate SU 1 038 377 discloses a steel alloy which is said to be resistant to stress corrosion, primarily in a chlorine-containing environment. However, stress corrosion involves substantially lower temperatures than those encountered in superheater applications. The alloy described in SU 1038377 contains (in weight %) 0.03-0.08 C, 0.3-0.8 Si, 0.5-1.0 Mn, 17-19 Cr, 9-11 Ni, 0.35-0.6 Mo, 0.4-0.7 Ti, 0.008-0.02 N, 0.01-0.1 Ce and the remainder Fe. The heat check resistance and weldability of the alloy are unsatisfactory.